Wastewater On-line Monitoring Systems: Acceptance, Operation and Data Validity under HJ 354, HJ 355 and HJ 356-2019

1. Four standards, four scopes - do not mix them up

On 2019-12-24 the Ministry of Ecology and Environment approved and issued eleven national environmental protection standards in a single announcement. Four of them form a closed loop for wastewater source on-line monitoring: HJ 353-2019 for installation, HJ 354-2019 for check and acceptance, HJ 355-2019 for operation and HJ 356-2019 for data validity determination. All four took effect on 2020-03-24, replacing the four 2007 trial specifications HJ/T 353, 354, 355 and 356, and all four are first revisions. The scope clause of HJ 355 is explicit: it applies to systems that have passed acceptance under HJ 354. HJ 356 governs the validity determination flow, indicators, methods, calculation of valid averages and the treatment of invalid data. The drafting bodies differ (HJ 354 by the China National Environmental Monitoring Centre with the Hebei and Hunan provincial centres; HJ 355 by the China National Environmental Monitoring Centre with the Jiangxi provincial centre; HJ 356 by the China National Environmental Monitoring Centre with the Hubei provincial centre and the Chinese Research Academy of Environmental Sciences), so small divergences in wording between the texts are not surprising. One measured divergence is set out in section 5.

2. Acceptance: preconditions first, then indicators

2.1 Seven preconditions under clause 4.1 of HJ 354-2019

• Installation, commissioning and trial operation have been completed under HJ 353, with all indicators meeting Table 3 of HJ 353-2019, and commissioning and trial operation reports submitted;
• Verification certificates for the flow meter and the standard measuring weir or flume are provided, together with evidence that the instruments meet the technical requirements of Table 1 of HJ 353-2019;
• The underlying communication network and protocol comply with HJ 212, with a self inspection report and a networking certificate issued by the competent ecology and environment authority;
• The automatic water sampling unit has been operating stably for one month, can collect instantaneous and representative composite samples, and can retain samples and raise alarms;
• The data control unit has been operating stably for one month, with equipment operation rate above 90 percent and data transmission rate above 90 percent, calculated by the two formulae in clause 4.1.7;
• Power supply is uninterrupted throughout the acceptance process.

2.2 Quantified requirements for the station building and sampling

Clause 5.3 sets measurable floors for the monitoring station: a new station must have a floor area of not less than 15 m2 and a height of not less than 2.8 m, be located not more than 50 m from the sampling point, be provided with an electrical load of not less than 5 kW with a stabilised supply, be sealed and fitted with heating and cooling air conditioning that restarts automatically after a power cut, plus an exhaust fan, and be covered by video surveillance inside the station and at the sampling port. Clause 5.4 requires the automatic sampling unit to have a manual comparison sampling port for composite samples, to run exposed pipework marked with flow direction, to use PVC or PPR rigid pipe that does not affect analytical results, and to place the sampling port at the centre of the flow path at the head of the standard measuring weir or flume with the port facing downstream; where combined discharge is measured, sampling must take place where the flows are fully mixed.

2.3 Instrument performance acceptance indicators (Table 2)

• 24 h drift at 80 percent of the upper limit of the working range: plus or minus 10 percent of full scale for CODCr, TOC, ammonia nitrogen, total phosphorus and total nitrogen analysers, and plus or minus 0.5 for pH analysers;
• Accuracy: for CODCr and TOC, plus or minus 5 mg/L where the certified reference solution is below 30 mg/L and plus or minus 10 percent at or above 30 mg/L; for ammonia nitrogen, plus or minus 0.3 mg/L below 2 mg/L and plus or minus 10 percent at or above 2 mg/L; for total phosphorus, plus or minus 0.06 mg/L below 0.4 mg/L and plus or minus 10 percent at or above 0.4 mg/L; for total nitrogen, plus or minus 0.3 mg/L below 2 mg/L and plus or minus 10 percent at or above 2 mg/L; plus or minus 0.5 for pH;
• Actual sample comparison: for CODCr, plus or minus 5 mg/L below 30 mg/L (tested with a 20 to 25 mg/L standard solution in place of the actual sample), plus or minus 30 percent from 30 to 60 mg/L, plus or minus 20 percent from 60 to 100 mg/L and plus or minus 15 percent at or above 100 mg/L; for ammonia nitrogen, plus or minus 0.3 mg/L below 2 mg/L (with a 1.5 mg/L certified reference material) and plus or minus 15 percent at or above 2 mg/L; for total phosphorus, plus or minus 0.06 mg/L below 0.4 mg/L (with a 0.3 mg/L certified reference material) and plus or minus 15 percent at or above 0.4 mg/L; for total nitrogen, plus or minus 0.3 mg/L below 2 mg/L and plus or minus 15 percent at or above 2 mg/L; plus or minus 0.5 for pH;
• Ultrasonic open channel flow meter: level comparison error 12 mm and flow comparison error plus or minus 10 percent over a 10 minute cumulative volume; automatic water sampler: sampling volume error plus or minus 10 percent and temperature control error plus or minus 2 degrees C.

One point deserves attention: clause 6.3.1 requires the portable open channel comparison device used for level comparison at acceptance to have a level measurement accuracy of 0.1 mm or better, whereas clause 8.4.2.2 of HJ 355-2019 requires only 1 mm or better for field comparison during operation. The same indicator is strict at acceptance and looser in operation, so procurement and maintenance specifications must be checked against each stage separately.

3. Operation: range setting, sampling frequency and quality control cycles

Clause 5.1.1 of HJ 355-2019 is easily overlooked yet has wide consequences: the upper limit of the measuring range shall be set to two to three times the discharge standard limit applied at the site; if the actual concentration exceeds the configured range, manual monitoring under clause 9.7 applies. Clause 5.2.2 requires access privileges for instrument operation and parameter modification, requires the parameters before and after any change to be recorded on paper or electronically, requires an unalterable entry in the instrument running log, and requires retention for at least one year.

Sampling is split by instrument type under clause 6.1. pH analysers, thermometers and flow meters measure instantaneous samples: for continuous discharge at least one data point every 10 minutes, and for intermittent discharge not fewer than six times the number of cumulative discharge hours. CODCr, TOC, ammonia nitrogen, total phosphorus and total nitrogen analysers measure composite samples: for continuous discharge each hour from midnight forms a sampling period, with the automatic sampler collecting time proportional or flow proportional samples (the text gives the example of one sample every 15 minutes, four per hour), and the result recorded as the average concentration for that period. Clause 6.2.2 requires data to be reported to the platform within 10 minutes of the determination, requires instrument and data logger storage of at least one year of data, and requires transmission under HJ 212 with automatic retransmission of failed records.

The quality control cycle is the backbone of operation under chapter 8: automatic standard solution checks at intervals not exceeding 24 h, calibration at intervals not exceeding 168 h, using a standard solution at roughly 0.5 times the upper limit of the field working range; if automatic calibration and verification exceed 6 h, manual monitoring applies with not fewer than four reports per day and intervals not exceeding 6 h; actual sample comparison at least monthly for CODCr, TOC, ammonia nitrogen, total phosphorus and total nitrogen, and at least monthly for pH and temperature; ultrasonic open channel flow meters at least quarterly; and the valid data rate on a monthly cycle shall not be below 90 percent. On maintenance, clause 9.1 requires approval by the competent ecology and environment authority before shutdown, removal, replacement or restart, and clause 9.2 requires reporting within 24 h where force majeure or an emergency stops the system.

4. Data validity: three red lines and two substitution rules

Chapter 4 of HJ 356-2019 divides operating status into normal sampling periods and abnormal sampling periods. The latter covers instrument shutdown, fault repair or maintenance, and calibration or verification; any data output during those periods is invalid regardless of value. Clause 6.2 sets the criteria: where flow is zero the reported value is invalid; where the discrepancy between the analyser, the data acquisition and transmission unit and the platform received data exceeds 1 percent, the platform data are invalid; and where a standard solution test or an actual sample comparison fails, all on-line data from the moment of failure back to the moment of the last successful calibration or verification are invalid.

Chapter 7 sets the statistical basis: the valid daily average is a flow weighted mean, and the number of valid data points included shall not be less than 75 percent of the number of data points that should have been obtained that day; the valid monthly average is the arithmetic mean of the valid daily averages, with the number of valid daily averages not less than 75 percent of those that should have been obtained for the month. Data judged valid but below the detection limit must be corrected before being used, with correction values of 2 mg/L for CODCr, 0.01 mg/L for ammonia nitrogen, 0.005 mg/L for total phosphorus and 0.025 mg/L for total nitrogen. Note carefully the difference in scope from HJ 355: the 90 percent in clause 8.5 of HJ 355-2019 is a valid data rate whose denominator is the number of data points that should have been obtained in the month, whereas the 75 percent in clauses 7.2 and 7.3 of HJ 356-2019 is a floor on the sample count used to compute a daily or monthly average. They constrain different things; neither substitutes for the other, and meeting the 90 percent rate does not by itself establish a valid daily average.

Chapter 8 sets the substitution rules: where a valid daily average cannot be computed, the pollutant daily load may be substituted by the maximum of the 30 valid daily loads preceding the last successful calibration or verification, while concentration and flow are not substituted; for abnormal periods, manual monitoring takes priority, with not fewer than four manual measurements per day; and invalid data recorded when flow is zero are not substituted at all.

5. One textual divergence and items that must be confirmed elsewhere

A clause by clause comparison of Table 2 of HJ 354-2019 with Table 1 of HJ 355-2019 shows that the low concentration criterion for total phosphorus is not consistent. The acceptance standard (HJ 354) requires, for actual sample total phosphorus below 0.4 mg/L, substitution with a 0.3 mg/L certified reference material and a limit of plus or minus 0.06 mg/L; the operation standard (HJ 355) substitutes a 0.2 mg/L standard solution with a limit of plus or minus 0.04 mg/L. All other bands for CODCr, ammonia nitrogen, total nitrogen and pH agree. Engineering and operation documents should therefore state explicitly which standard number and which stage applies, so that a single instrument is not judged against two different criteria at acceptance and during operation.

Beyond this, neither HJ 355 nor HJ 356 prescribes the measurement principle or any brand threshold for instrument selection, nor detailed values such as the lightning protection earthing resistance of the station or the resolution of video surveillance. Those must be confirmed separately against HJ 353-2019 (installation), the approved environmental impact assessment for the project and the networking requirements of the local ecology and environment authority. The published files themselves are labelled released drafts, and the official texts published by China Environmental Publishing Group govern.